Certain wireless communication networks employ control channels that lack direct reception quality feedback, as would normally be used for link adaptation on such channels. Long Term Evolution (LTE) networks provide a ready example, as do WiMax networks and certain others.
As a specific example in the LTE context, user equipment does not send acknowledgment information for signaling received from an eNodeB on the Physical Downlink Control Channel (PDCCH). The lack of acknowledgments means that it is not easy to detect whether information sent over the PDCCH is successfully received by a user terminal. Examples of such information include uplink and downlink grants, which are transmitted in LTE as part of user scheduling operations.
Lacking direct feedback for the PDCCH, LTE provides an algorithm for “outer loop” control of the coding rate for the PDCCH. Feedback received for transmissions sent on the Physical Downlink Shared Channel (PDSCH) drives the algorithm. Consequently, with this approach, the eNodeB corrects the Signal-to-Noise-and-Interference Ratio (SINR) of the PDCCH, responsive to channel quality feedback received for the PDSCH.
Adapting the PDCCH according to PDSCH feedback works well, at least to the extent that reception conditions are about the same for the PDCCH and the PDSCH. However, it is more “normal” for the reception conditions to differ between the PDCCH and the PDSCH. For example, while these channels may experience the same path loss, they likely will experience different levels/kinds of interference, given that they use different channel resources. Thus, driving the PDSCH outer loop control with PDCCH feedback results in suboptimal adaptation of the PDSCH, to the extent that channel conditions for the PDSCH and PDCCH diverge. In turn, sub-optimal adaptation of the PDCCH leads to network inefficiency, among other problems.